The suffix ۋى|ىي may be added to a modest but significant number of nouns (usually of Persian/Arabic, but sometimes Russian, origin) to create adjectives that relate to the noun in question. Though it may seem baffling to call ۋى and ىي one suffix, the former was originally ۋىي - the version attached to nouns ending in vowels - before being subjected to spelling reforms (though the lost ي is often retained in speech). In modern written Uyghur, ۋى is found always when the noun ends in a short vowel, with (short) ئا|ئە reducing to an ئى.

people/community

[noun-to-adjective]

public

time (obsolete variant)

[noun-to-adjective]

modern

meaning/sense

[noun-to-adjective]

spiritual

theory

[noun-to-adjective]

theoretical

tradition

[noun-to-adjective]

traditional

views/ideology

[noun-to-adjective]

ideological

neighborhood

[noun-to-adjective]

local

the Party

[noun-to-adjective]

Party (adjective)

people, community + [adjective] = public

ئامما +ۋى =ئاممىۋى

time + [adjective] = modern

زامانە +ۋى =زامانىۋى

meaning, sense + [adjective] = spiritual

مەنە +ۋى =مەنىۋى

theory + [adjective] = theoretical

نەزەرىيە +ۋى =نەزەرىيىۋى

tradition + [adjective] = traditional

ئەنئەنە +ۋى =ئەنئەنىۋى

views, ideology + [adjective] = ideological

ئىدىيە +ۋى =ئىدىيىۋى

neighborhood + [adjective] = local

مەھەللە +ۋى =مەھەللىۋى

the Party + [adjective] = Party (adjective)

پارتىيە +ۋى =پارتىيىۋى

public

on bus

person

few

time as well

there exists?

Does there also exist a time when the public buses are not full of people?

ئاممىۋىئاپتوبۇستائادەمئازۋاقىتمۇبارمۇ؟

When the final vowel is a long vowel and does not reduce, both ۋى and ىي are possible. No simple rules for memorizing which makes sense where exist, so it's best to just memorize these words (there aren't many).

sacrifice/offering

[noun-to-adjective]

voluntary

origin/genesis

[noun-to-adjective]

primitive/original

world (noun)

[noun-to-adjective]

world (adjective)

sacrifice, offering + [adjective] = voluntary

پىدا +ىي =پىدائىي

origin, genesis + [adjective] = primitive, original

ئىپتىدا +ىي =ئىپتىدائىي

world (noun) + [adjective] = world (adjective)

دۇنيا +ۋى =دۇنياۋى

The ىي is attached to the nouns ending in consonants. In the vast majority of the cases when the penultimate letter in the noun is an ئا or an ئە, no reduction takes place as almost all such vowels are long.

public (noun)

[noun-to-adjective]

public (adjective) / general

one's choice

[noun-to-adjective]

voluntary / of one's own free will

eternity

[noun-to-adjective]

eternal

etiquette

[noun-to-adjective]

literary

history

[noun-to-adjective]

historical

individual

[noun-to-adjective]

private/personal

law

[noun-to-adjective]

legal

ancient times

[noun-to-adjective]

ancient

midway / middle of

[noun-to-adjective]

semi-

creation/invention

[noun-to-adjective]

creative/inventive

economy

[noun-to-adjective]

economic

morals

[noun-to-adjective]

moral (adjective)

nerves

[noun-to-adjective]

nervous/tense

sex (gender)

[noun-to-adjective]

sexual

thought/idea

[noun-to-adjective]

imaginary

spirit

[noun-to-adjective]

spiritual

satire

[noun-to-adjective]

satirical

humanity

[noun-to-adjective]

human (adjective)

religion

[noun-to-adjective]

religious

class (in society)

[noun-to-adjective]

class (adjective)

public (noun) + [adjective] = public (adjective), general

ئومۇم +ىي =ئومۇمىي

one's choice + [adjective] = voluntary, of one's own free will

ئىختىيار +ىي =ئىختىيارىي

eternity + [adjective] = eternal

ئەبەد +ىي =ئەبەدىي

etiquette + [adjective] = literary

ئەدەب +ىي =ئەدەبىي

history + [adjective] = historical

تارىخ +ىي =تارىخىي

individual + [adjective] = private, personal

شەخس +ىي =شەخسىي

law + [adjective] = legal

قانۇن +ىي =قانۇنىي

ancient times + [adjective] = ancient

قەدىم +ىي =قەدىمىي

midway, middle of + [adjective] = semi-

نىسپ +ىي =نىسپىي

creation, invention + [adjective] = creative, inventive

ئىجاد +ىي =ئىجادىي

economy + [adjective] = economic

ئىقتىساد +ىي =ئىقتىسادىي

morals + [adjective] = moral (adjective)

ئەخلاق +ىي =ئەخلاقىي

nerves + [adjective] = nervous, tense

ئەسەب +ىي =ئەسەبىي

sex (gender) + [adjective] = sexual

جىنس +ىي =جىنسىي

thought, idea + [adjective] = imaginary

خىيال +ىي =خىيالىي

spirit + [adjective] = spiritual

روھ +ىي =روھىي

satire + [adjective] = satirical

ھەجۋ +ىي =ھەجۋىي

humanity + [adjective] = human (adjective)

ئىنسان +ىي =ئىنسانىي

religion + [adjective] = religious

دىن +ىي =دىنىي

class (in society) + [adjective] = class (adjective)

سىنىپ +ىي =سىنىپىي

However, there does exist a small handful of examples where the final ئا or ئە is short and thus reduces to an ئى.

leader

[noun-to-adjective]

leading

center

[noun-to-adjective]

central

leader + [adjective] = leading

رەھبەر +ىي =رەھبىرىي

center + [adjective] = central

مەركەز +ىي =مەركىزىي

A particularly useful set of words to remember here are the four compass directions.

east (noun)

[noun-to-adjective]

east (adjective)

north (noun)

[noun-to-adjective]

north (adjective)

west (noun)

[noun-to-adjective]

west (adjective)

south (noun)

[noun-to-adjective]

south (adjective)

east (noun) + [adjective] = east (adjective)

شەرق +ىي =شەرقىي

north (noun) + [adjective] = north (adjective)

شىمال +ىي =شىمالىي

west (noun) + [adjective] = west (adjective)

غەرب +ىي =غەربىي

south (noun) + [adjective] = south (adjective)

جەنۇب +ىي =جەنۇبىي

A certain set of nouns ending in a consonant-vowel-consonant will lose the final vowel with the addition of the ىي. All of these belong to the group of special nouns that also lose the final vowel when taking possessive suffixes.